The demand for processing power in computer systems is increasing. Increasing the processing power of a computer system often results in an increase in the power consumed by the computer system. However, for computer systems implemented on user devices, such as mobile smart phones and tablets, it is important to keep the power consumption of the computer system at a low level because, for example, the power supply to the user device may be limited. For example, the user device may be battery-operated, in which case it is particularly important to prevent the processing power from becoming too high in order to ensure that the battery life of the user device is not reduced to an unacceptable level by the implementation of the processor on the user device.
In order to address the issue of the trade-off between higher processing power and lower power consumption, a computer system may include two (or more) sub systems, which each implement a separate processor. The multiple processors operate independently, for example performing specialized tasks in the computer system. For example, a user device such as a smart phone or tablet may implement a first subsystem including a baseband processor and a second subsystem including an application processor (AP). The baseband processor acts as a Radio Frequency (RF) modem thereby processing data for communication between the user device and a radio network. The application processor executes an operating system of the user device and handles other multimedia features on the user device, and for example processes data relating to peripherals of the user device such as a display, a Wi-Fi module, a GPS module, etc.
Communication between the two sub-systems (i.e. between the processors of the two subsystems) is referred to as Inter Processor Communication (IPC). “IPC activities” are communications between the two processors. The IPC activities convey various types of information, spread over several communication channels, including:                control information (e.g. for initiating a voice call between the user device and another node of the radio network);        data (e.g. Internet Protocol (IP) data for transmission between the user device and another node of the radio network);        logging information; and        file system information (e.g. when the baseband file system is physically located on AP-managed Flash memory).        
Each processor may operate in one of a plurality of possible operational modes. For example, the Application Processor may operate in an awake mode in which it is able to process IPC activities that it receives from the baseband processor. The Application Processor may alternatively operate in a sleep mode in which it does not process IPC activities. Typically the Application Processor will consume less power when it operates in the sleep mode compared to when it operates in the awake mode. Therefore, when no IPC activities are being communicated between the processors, the Application Processor may be configured to operate in the sleep mode to conserve power. Every time a quantum of data is sent over the IPC from the baseband processor to the Application Processor, the Application Processor needs to be in, or enter into, a state which allows for that communication to happen. If the Application Processor is in the sleep mode when the IPC activity is initiated then it is “woken up”, i.e. switched to operate in an awake mode in order to process the IPC activity. As an example, the awake mode may have a power consumption which is greater than that of the sleep mode by a factor of approximately 50. It is beneficial to minimize the power consumed by the computer system.